Appliance theft prevention alarm system

ABSTRACT

A theft prevention alarm system for use with an electrical appliance which includes a transmitter located within the appliance and which continuously generates an electrical signal of a predetermined frequency over the alternating current power supply lines. A receiver is connected to the alternating current power supply and operates an alarm system when the electrical signal having the predetermined frequency can no longer be detected.

nited States Patent lnventors Stephen M. Curry 5627 Monticello Ave., Dallas, Tex. 75206; Philip A. Williams, 1624 Highland Road,

Dallas, Tex. 7521s Appl. No. 863,196 Filed on. 2, 1969 Patented Jan. 4, 1972 APPLIANCE THEFI PREVENTION ALARM SYSTEM 1 Claim, 2 Drawing Figs.

US. Cl. 1. 340/280, 340/256, 340/409, 340/310 Int. Cl G08b 13/14 Field of Search 340/310, 280, 276, 256, 216, 416, 409, 224; 325/101, 102, I 364 TRANSMITTER [56] References Cited 7 UNITED STATES PATENTS 3,135,951 6/1964 Byrne 340/310 3 ,150,294 9/ 1 964 Dastidar 340/409 3,234,543 2/ 1966 Thompson et al. 340/310 3,411,150 11/1968 Schulein 340/280 3,462,756 8/ 1969 Mills 340/216 3,482,243 12/ 1 969 Buchsbaum 340/3 10 Primary Examiner-Thomas B. Habecker Assistant Examiner--Glen R. Swarm, Ill Attorney-Richards, Harris and Hubbard ABSTRACT: A theft prevention alarm system for use with an electrical appliance which includes a transmitter located within the appliance and which continuously generates an electrical signal of a predetermined frequency over the alternating current power supply lines. A receiver is connected to the alternating current power supply and operates an alarm system when the electrical signal having the predetermined frequency can no longer be detected.

PATENTED m 4 I972 FIG. I

RECEIVER TRANSMITTER FIG. 2

INVENTORS S. M.CURRY P. A.W|LLIAMS The general object of this invention is to provide an alarm system to protect electrical appliances from theft. Another object is to provide-a system which is sufficiently sophisticated to foil even the most cleverthief andyet is sufficiently simple to install that it can be installed and used by the average person in his home. Another objectis to provide a system which can be built intoan appliance by themanufacture and'marketed with the detectionand alarm circuit described herein to provide "a built-in theft'protection system.

The system described herein differs from and is an improvement over the prior art (c.f. J. Schulein, U.S. Pat. No. 3,41 1,150) in the following respects:

1. Since the transmitter described herein is situated within the appliance to be protected,'the appliance can be plugged into any electrical outlet; This is clearly more desirable than a system such as that of Schulein'(in which a transmitter is connected to a special wall outlet) because such a system protects only theappliance plugged into the specialwall outlet. The system described herein thus allows the appliances to be protected to be relocated without the necessity of reinstalling the transmitter.

2. The installation of the transmitter within the cabinet of an appliance is sufficiently simple that it can be performed by the average person with no special knowledge of electronics. This system can therefore be made available to the general public, whereas other systems, requiring modifications to household wiring, must be installed by experts.

3. Since the transmitter is out of sight within the cabinet of the appliance, there is no unsightly and revealing evidence near the appliance indicating that it is protected.

4. Since the receiver described herein is designed to work with batteries if the user so desired, the alarm system will function even if the thief attempts to foil'the alarm by shorting out the household power circuit.

The system consists of two basic units, transmitter and receiver. The transmitter unit is placed within the cabinet of an electrical appliance and draws its power from the power cordthat serves the appliance. Thedevice transmits a lowlevel electricalsignal with'a frequency in the range of 20 to 400 kilocycles along the power cord which feeds the device. This signal is not transmitted throughthe air, and it is blocked by transformers. Therefore, its range is limited to the approximately six houses surrounding the stepdown transformer on the utility pole. The unit is transistorized and operates continuously as long as the appliance to which it is attached is plugged into a walloutlet.

The receiving unit is plugged into a wall outlet. It detects the signal from the transmitter with a tuned circuit, amplifies it, and uses the amplified signal to operate a relay. While the receiver is receiving a signal from the transmitter, the relay is open. If the receiver ceases to detect a signal from the transmitter, the relay closes immediately. The relay, when closed, causes a standard burglar alarm bell to sound. A switch is provided on the unit which allows the user to select either of two operational modes:

1. Auto reset: The alarm will cease to sound if the transmitter is reactivated (i.e., by plugging the appliance to which it is attached back in).

2. Manual reset: The alann cannot be stopped, once started, except by setting the'mode switch on the receiver to auto."

The transmitters and receivers would be produced in pairs, each pair consisting'of one transmitter and one receiver. Each pair might be tuned to operateat a slightly different frequency within the specified frequency range. This is'to insure that the transmitter of one pair would not function with the receiver of another pair. (If it were not for this feature, it would be possible for a thief to buy a transmitter and exchange it for the one 3 attached to the object of the theft, thereby foiling the alarm system.)

Two accessories would be used in conjunction with the transmitter. The television power plug accessory provides power to the transmitter enclosed in'the television cabinet from the power socket on the back of the television'chassis All televisions are provided with a safety'in'terlock which disconnects the power to the set when the backis removed. This accessory is plugged into the back of the chassis, andthe cabinet back is plugged'into it. The accessory deli'v'ers power to the transmitter. 1f the back of thetelevisionis removed; the" power to the transmitter is automatically cutoff.

The appliance power patch cord accessory is for usewith" any electrical appliance. It is a pow'er cord terminated at one end by a female jack connectingt'o the transmit'tenarid at'the other end by two small-alligator clips. The"transrnitter"is placed in the cabinet of theappliance, audit is attached to'the" power source by means of the alligator clips on the patch cord.

The transmitter (FIG. 2) consists of a power" supply l'-3 which drives an oscillator circuit 4-8," arida capacitive coupling circuit 9-10 to couple the RF signal back into the AC power lines. Power is supplied to thetransmitter through'the accessory plug 0.

The transformer l stepsdown the voltagetroni the 'AC" power line and isolates the circuit from the AC line; The reduced voltage is rectified-to DC by the run-wavereenfir 2, and the AC ripple is partially filtered out by 3.

Initially, an electron current flows through the transformer windings 6 and through'the collector of the"transi st'or 4Yto ground. The transistor base bias is applied by means of are sistor 5. The current in 6 inducer an EMF in 7, resultingin a reduction of the base current of the'transis to'r. The reduced current causes the collector current to drop, reducing the'cu'r' rent through 6. The decrease in the current th'r'ou gh 6'ind trces an EMF of opposite polarity in 7, thereby increasing thebas'e current. The increased base current causes an increase inthe" current through the collector and 6.'Thi's process then repeats itself as long as the power is supplied, so thatan oscillating' current is set up in 6. The frequency of the oscillationsis determined by the tuned circuit'8.'

The oscillating current in 6 induces an oscillating EMF in' the secondary windings 9 of the oscillator transformer. This oscillating signal is coupled throughthe couplingcapacitors' 1 back into the AC power line.

The receiver FIG. 1 consists of a powe'r'supply- 1-3, a dete'c-- tor circuit 4-7, and a two-transistor amplifiei' 'circuit 8-12- which operates a relay 13. Power is supplied to the" receiver directly from the AC line by means of the s'taiidard'AC line plug 0.

The transformer 1 steps down thefvoltage 'fr'ornthe' AC power line and isolates the circuit from the'AC lineJThe' reduced voltage is rectified to DC by'the full-wav'e'rectifier 2 and the AC ripple is partially filtered'out by 3.'The highfrequency signal from the transmitter is capacitativelycoiipled' to the primary of the RF transformer 4. Thtuned circuit 5 is adjusted by varying the capacitor to resonate at the frequency 0f the transmitter. The oscillating current induced in' the resonant circuit 5 is rectified by the diode6, and the ripple 'isfiltered out by 7.

The weak rectified signal from the detector circuit is applied to the base of the transistor 8,"whi'cli is biased by means" of resistors 9l0.'The transistor 8 amplifies the weak'aeteer r current, and the amplified'current through the emitter-colicctor circuit of 8 is applied to' the base of 12, which further'arn-i plifies the current. The current from the two-transistor ampli-' fier actuates a relay 13 which brea'ks thecircuit leading to the" alarm. If the signalfrom the transmitter should cease, thecur rent in the relay would be greatly reduced, so that relaywould close and complete the alarm circuit, thereby causing the alarm to sound.

The latching relay 14 is used to cause'the alarrri'to continue to sound even if the transmitter is reactivated.To use the latching relay, the mode selector switch 15 is'closed maririially again. 16 is where receiver attaches to bell transformer or to batteries, and 17 is where receiver attached to alarm bell or buzzer.

It is desired to secure by Letters Patent:

1. A theft prevention alarm system for use with an electrical appliance comprising:

a transmitter including a power plug for connection within the case of an electrical appliance to the alternating current power cord of the electrical appliance,

said transmitter including an oscillator operable by electrical power supplied through the power cord of the appliance to generate an electrical signal of a predetermined frequency,

a transformer connected between said oscillator and the power cord to couple said electrical signal into the alternating current power supply,

a receiver including an isolation transformer for connection into the alternating current power supply remote from said appliance,

a rectifier connected to said isolation transformer for providing DC bias voltage for said receiver,

a tuned circuit tuned to said predetermined frequency,

a second transformer for coupling said electrical signal having the predetermined frequency to said tuned circuit,

amplifier means for amplifying the output from said tuned circuit,

switch means operable by the output from said tuned circuit, and

an alarm system operable in response to said switch means to provide an alarm indication when said receiver ceases to detect said predetermined frequency electrical signal. 

1. A theft prevention alarm system for use with an electrical appliance comprising: a transmitter including a power plug for connection within the case of an electrical appliance to the alternating current power cord of the electrical appliance, said transmitter including an oscillator operable by electrical power supplied through the power cord of the appliance to generate an electrical signal of a predetermined frequency, a transformer connected between said oscillator and the power cord to couple said electrical signal into the alternating current power supply, a receiver including an isolation transformer for connection into the alternating current power supply remote from said appliance, a rectifier connected to said isolation transformer for providing DC bias voltage for said receiver, a tuned circuit tuned to said predetermined frequency, a second transformer for coupling said electrical signal having the predetermined frequency to said tuned circuit, amplifier means for amplifying the output from said tuned circuit, switch means operable by the output from said tuned circuit, and an alarm system operable in response to said switch means to provide an alarm indication when said receiver ceases to detect said predetermined frequency electrical signal. 